If the data to be converted is available only in sequential blocks (such as data read from a stream) or if the amount of data is so large that it needs to be divided into smaller blocks, you should use the Decoder or the Encoder provided by the GetDecoder method or the GetEncoder method, respectively, of a derived class.

The GetByteCount method determines how many bytes result in encoding a set of Unicode characters, and the GetBytes method performs the actual encoding. The Encoding.GetBytes method expects discrete conversions, in contrast to the Encoder.GetBytes method, which handles multiple conversions on a single input stream.

Several versions of GetByteCount and GetBytes are supported. The following are some programming considerations for use of these methods:

Your app might need to encode many input characters to a code page and process the characters using multiple calls. In this case, you probably need to maintain state between calls, taking into account the state that is persisted by the Encoder object being used. (For example, a character sequence that includes surrogate pairs might end with a high surrogate. The Encoder will remember that high surrogate so that it can be combined with a low surrogate at the beginning of a following call. Encoding won't be able to maintain the state, so the character will be sent to the EncoderFallback.)

If your app handles string inputs, you should use the string version of GetBytes.

The Unicode character buffer version of GetBytes(Char*, Int32, Byte*, Int32) allows some fast techniques, particularly with multiple calls using the Encoder object or inserting into existing buffers. Bear in mind, however, that this method version is sometimes unsafe, since pointers are required.

If your app must convert a large amount of data, it should reuse the output buffer. In this case, the GetBytes version that supports byte arrays is the best choice.

Consider using the Encoder.Convert method instead of GetByteCount. The conversion method converts as much data as possible, and does throw an exception if the output buffer is too small. For continuous encoding of a stream, this method is often the best choice.